Microbial degradation and its influence on components of coalbed gases in Enhong syncline, China

Coalbed gases (CBG) in Enhong syncline are characterized by high concentration of C2+ (C2–5), with the highest content of ethane over 30%. However, the concentrations of C2+ are not evenly distributed in the syncline. Based on the analysis of δ13C1, δ13C2, δ13C3, δ13CO2, δDCH4 of CBG and their origin diagrams in the normal and abnormal areas, this research shows that gases in both areas are thermogenic gases and the reason for the uneven distribution of C2+ is that the microbial degradation action on gases is stronger in the normal area than in the abnormal area. The secondary biologic gases in the normal area are mainly characterized by that the carbon isotopes become obviously lighter in methane and become heavier in ethane, whereas the molecular and isotopic compositions of CO2 change little. These features indicate that the secondary biologic gases are mainly generated by the microbial degradation of C2+, not generated by the reduction of CO2. The degradation process is selective to make the residual ethane being enriched in 13C and the generated methane rich in 12C.